264 PEOCEBDINGS OF SECTION C. 



9. — Hornblende andesite from Omaru Island, Coromandel. 

 This rock difieis in appearance from all other hornblende andesites 

 that I have seen, in that it is black, and has a semi- vitreous lustre. 

 It looks like an augite andesite, but the hornblende is distinct, 

 though small in quantity. The ground-mass is not abundant and 

 is made up of small felspar laths with a dirty brown substance 

 (opacite) between them. The felspars are pliagioclase shewing 

 twinning on both albite and pericline types, and are strongly 

 zoned with negative cystals. Brown hornblende occurs sparingly 

 and is sometimes changed into a pleochroic chlorite which is 

 bordered with brown o^Dacite. Magnetite is in rather large grains 

 and most of it is decomposing into limonite. The rock is, I think, 

 a dyke. S.G. =2.646. 



Although I collected every variety of rock that I could find at 

 the Thames, including specimens of the Miocene dolerites of the 

 Geological Survey, I have failed to find any sharp line of division. 

 If two, widely separated, volcanic formations are present at the 

 Thames they cannot be recognized by mineialogical characters. 

 The absence or piesence of pyrites would quite fail as a test and 

 no one has supposed this to constitute a diiference. Chlorite 

 ofiers a better chance, because if this mineral was only formed at 

 depths, it would not occur in a newer and superficial formation 

 but, under these conditions, neither would it occur in the upper 

 parts of the auriferous series, so that although its presence might 

 indicate the older series, its absence from a rock would be no proof 

 that the rock belonged to a newer series unless it could be shewn 

 that the whole of the upper beds of the older series had been 

 removed by denudation. Of this I have seen no stratigraphical evi- 

 dence and can find nothing unrlerthis head in the published reports 

 that appears to me to have any importance. If there wei'e two 

 widely separated series, as supposed, we shovild expect to find that 

 all or nearly all the bisilicates of the older series had been altered 

 into chlorite, etc. ; while in the newer series only the ordinary 

 series of charges would have taken place and there would be 

 no chlorite. But if tliere was only one series, we should then 

 expect to trace a gradual change in tlie rocks from those most 

 altered to those in which the alteration of bisilicates into chlorite 

 had only just commenced : and this is precisely what we do find. 



Mr. Cox says that the younger volcanic series is undoubtedly 

 distinct from the auriferous series because it is found at very 

 different levels; sometimes at 1500 feet or more, at other times 

 on spurs which are comparatively low lying, not more than three 

 hundred feet above the level of the sea {I.oc. cit., p. 20), and fi'om 

 this he infers "that the auriferous rocks had been deposited, 

 upheaved, fractured, and partially denuded — indeed so much so that 

 the conformation of the country corresponded more or less with that 

 which at present exists — before the younger rocks were deposited 

 upon them." I am sorry that I cannot agree with Mr. Cox here. 



